Asphalt is a commonly used material for construction purposes, such as a road pavement or roofing material. Asphalt alone, however, often does not possess all the physical characteristics desirable for many construction purposes. For instance, unmodified asphalt may exhibit a poor Performance Grade Rating (PG Rating) as a road pavement material. As used herein, PG Rating is defined as the average seven-day maximum and the single-day minimum pavement design temperature, wherein the maximum is determined 20 mm below the surface of the pavement and the minimum is determined at the surface of the pavement. Although the PG Rating for asphalt may widely vary, asphalt generally used in road pavement applications exhibit PG Ratings of about 64–22, which indicates a 64° C. average seven day maximum and a −22° C. single day minimum pavement design temperature.
When used as a road pavement material, asphalt is typically subjected to temperatures in excess of 64° C. twenty mm below the pavement surface and below −22° C. at the pavement surface. Temperatures outside this range lead to deterioration of the asphalt. pavement. Hence, it has for some time been an objective to broaden the PG Rating range of asphalt used in road pavement applications.
To broaden the PG Rating range of the asphalt pavement, modifiers are added to the asphalt. In addition to increasing the PG Rating range of the asphalt, modifiers also can improve other qualities of the asphalt, such as its toughness, flexibility and wear characteristics. Typically, modifiers are added to molten asphalt and mixed for several hours to produce a modified asphalt. Then, crosslinking agents can be added. The modified asphalt is then routed to a mixer where aggregate is added to produce the hot mix asphalt (HMA). The hot mix asphalt is then taken to the construction site for use in paving equipment.
Three major problems associated with the performance of hot mix asphalts (HMA) pavements can be moisture susceptibility (stripping), permanent deformation (rutting, bleeding and shoving) and cracking (thermal and fatigue). There are a number of modifiers that can be added to the HMA mixture that can provide a solution to each of these problems individually. Liquid anti-strips and hydrated lime are additives that can be used to reduce moisture susceptibility problems. They alter the surface chemistry at the interface between the aggregate and the asphalt resulting in an improved bonding of the asphalt and aggregate. Polymers can be used to modify an asphalt to increase the high temperature stiffness of the HMA, which can reduce the probability of rutting, bleeding and shoving. Polymers may be selected and used to impart elastomeric properties which can reduce thermal and fatigue cracking by allowing the modified asphalt to undergo repeated strains with recovery. However, it is difficult to find a modifier that will provide a solution to these problems simultaneously especially one that does not require high shear blending or modified blending procedures.
The current methods used in the United States to make an asphalt hot mix for road paving involves melting the asphalt in a large heated tank and adding 2 to 6 percent of an elastomer. Typically, high dosages of elastomer are used globally, usually a synthetic rubber such as styrene butadiene styrene block copolymer (SBS). The SBS block copolymer dissolves slowly in the asphalt due to viscosity differences when only stirring is used. Tanks equipped with high shear stirrers shorten the time to dissolve the elastomer. After the elastomer is dissolved a crosslinking agent, such as sulfur, peroxide or a transition metal can be added to crosslink the elastomer. Adding the cross-linking agent, sulfur, at the same time as the SBS block copolymer results in immediate and localized cross-linking, which may manifest itself as agglomerated SBS clumps that do not dissolve quickly or easily. These clumps can require further blending/milling. The SBS block copolymer can actually form a matrix in the asphalt. If the crosslinking agent is added too soon to the mix, before the rubber has dissolved, an intractable mass of rubber is formed which is not dissolvable. If the crosslinking agent is added late, both time and money are then wasted. Hence, the addition of the crosslinking agent is critical to the correct make up and economics of the asphalt hot mix.
The use of sulfur to crosslink rubber has been known in the industry for about a hundred years. Much research has been concerned with adjusting the speed of the crosslinking reaction. That is, the development of accelerators, crosslink modifiers and retarders has been a very lucrative and fruitful area of research for the many chemical suppliers to the vulcanization industry. However, it should be noted that all modifications to the timing of sulfur as a crosslinking agent have resulted in shorter times to affect more crosslinking. Thus, no additive is available to prolong, delay or slow down the action of the sulfur crosslinking. Sulfur by itself is as slow as sulfur can be made to crosslink at a given temperature.
Cross-linking of the hot asphalt mix is not required, but highly preferred. Without cross-linking, more elastomer and elastomer of higher molecular weight are required to achieve the same end point, adding cost to the modification package. The use of a cross-linking agents permits the use of lower molecular weight elastomers, which are inherently easier to dissolve and may be lower cost. Sulfur, if used, is actually a cost reducer since it allows less elastomer to be used to achieve the same performance properties.
The addition of the elastomer to the tank of asphalt usually involves dumping a large quantity of elastomer into the top of the mix tank. When the sulfur is added similarly, a large disagreeable cloud of dust can be formed as well as fumes from the sulfur contacting the molten asphalt. In some cases, a hazardous sulfur cloud or sulfur gases can form and resulting odor may lead to the evacuation of the area until the cloud subsides. Even the addition of chunks of sulfur can lead to dust clouds and disagreeable odor.
The process of introducing finely divided dry materials, such as sulfur, into hot reaction vessels or tanks can present a hazard due to the formation and presence of sulfur dioxide or hydrogen sulfide vapors in combination with elevated processing temperatures. In addition, finely divided dry materials often do not mix efficiently when combined with the liquid material in the system. In an attempt to remedy this problem, the finely divided dry material can be pre-dispersed in oil. However, the tendency for the heavier finely divided material to settle requires constant agitation. More viscous oils and/or asphalt fluxes have been utilized to prevent the settlement of the finely divided material, but the use of these materials can require the application of higher processing temperatures to maintain the oil and or asphalt fluxes at a pumpable viscosity. In addition, this method can also require constant agitation to maintain dispersion of the finely divided material. As a result, the higher temperature needed in processing can increase the risks of emission of toxic gases, such as hydrogen sulfide, which is highly toxic and flammable.
Elastomers are more difficult to disperse in molten asphalt. They require the use of high shear mixing, increased processing temperatures, and long mixing times to obtain a good mix. Elastomers tend to increase the process viscosity of the asphalt. This increase in process viscosity can make the modified asphalts more difficult to mix and coat the aggregate in the mixing zone and subsequently more difficult to compact during installation of the hot mix asphalt composition. This is a technical and practical issue currently occurring in the industry. Typically, elastomers commonly used do not improve adhesion to aggregate in wet environments and can contribute to anti-stripping properties. These hot mix asphalt compositions still require the inclusion of an anti-stripping additive.
It is, therefore, apparent that there is a need for asphalt modifiers capable of modifying the physical characteristics of asphalt, as indicated by a broader PG rating. In addition, there is a need for modified asphalt compositions where the modifiers are easily dispersed in the asphalt. Furthermore, there is a need for modified asphalt compositions where there is reduction in dust and gas formation when the modifiers are added to the molten asphalt.